Kelly
loudspeakers



March 14, 1967 s. KELLY LOUDSPEAKERS Filed Aug. 25, 1965 United StatesPatent 3,308,904 LOUDSPEAKERS Stanley Kelly, Enfield, Middlesex,England, assignor to Decca Limited, London, England, a British CompanyFiled Aug. 23, 1965, Ser. No. 481,582

Claims priority, application Great Britain, Aug. 26, 1964,

4 Claims. (Cl. 181-32) This invention relates to loudspeakers.

The lower frequency response of a loudspeaker system depends on theresonant frequency determined by the acoustic stiffness of the enclosure(referred to the mechanical driving point of the loudspeaker unit), thecompliance of the loudspeaker restoring force and the mass of theloudspeaker diaphragm together With the air loading. It is well knownthat the low frequency response of a loudspeaker system can be extendedby providing what is commonly called a vented enclosure on one side ofthe diaphragm; such an enclosure has a port or tube through one wall ofthe enclosure so tuning the system in the form of a Helmholtz resonator.The port may be replaced by a lightly suspended diaphragm, the mass ofthis diaphragm being adjusted to that of the optimum reflected mass ofthe air vent; this improves the efficiency of the system in that theoverall effective volume of the enclosure is increased because of theabsence of the port or tube.

The present invention is directed towards further improving theefliciency of a loudspeaker system.

According to this invention, a loudspeaker comprises a piston driven byan electro-mechanical transducer whereby the piston is moved withrespect to a fixed part of the assembly in accordance with appliedelectrical signals, and an annulus surrounding the piston andcompliantly coupled to the piston and to said support, the annu- 1us,andcompliant coupling together forming an air seal between the piston andsupport and the annulus .being formed of a rigid element or a number ofseparate rigid elements having a mass and effective area to givemultiple resonances at or near the nominal low frequency resonance ofthe piston when measured in free air. By effective area is meant thearea normal to the direction of movement of the piston. It will beunderstood that the piston need not be circular and it may for examplebe elliptical; the annulus would be suitably shaped. The piston may be acone or it may be flat or of dish-shape.

The aforesaid support conveniently comprises a rigid frame constitutingpart of the loudspeaker housing assembly.

The compliant coupling may comprise a flexible material, e.g. a fabric,such as a doped linen sheet or an artificial fibre sheet, or a plasticfoil such as a polyvinyl chloride sheet, or a metal foil such as analuminium foil. Conveniently this flexible material extends over thewhole region between the piston and the support, the aforesaid annulusbeing fixed to the flexible material to provide the required mass andrigidity.

For optimum performance, the effective area and mass of the annulus aremade equal respectively to the effective area and mass of the piston;and the compliances of the piston to the support (C,,,,), the piston tothe annulus (C and the annulus to the support (C are so proportionedthat together with the reflected compliance of the enclosure formed bythe piston, annulus and support, the two resonant frequencies of thesystem are displaced from the nominal resonant frequency by 42 of thenominal resonant frequency of the piston (less the annulus mass Md andthe annulus to support compliance "ice" C when measured in free air. Itwill be seen that this system is analogous to a vented enclosureloudspeaker system but, in place of the port mass and air load, there isprovided the piston to annulus compliance C the annulus mass M and theair load. The introduction of the compliance c which has no counterpartin a vented enclosure system, increases the loading on the piston atfrequencies below resonance thereby preventing excessive excursion ofthe piston and consequential damage to the system.

Preferably the piston for the above described loudspeaker system is madeof an expanded thermo-set-ting resin having hard skinned surfaces and ofa density between 1 and 3 lb. per cu.-ft. as described in thespecification of copending application Ser. No. 481,622, filed August23, 196.5. Preferably also the piston and enclosure are arranged so thatthe piston area is related to the volume of the enclosure in the mannerset forth in the specification of copending application Ser. No.481,581, filed August 23, 1965.

- In the accompanying drawings:

FIGURE 1 is a diagrammatic illustration of one construction ofloudspeaker;

FIGURE 2 is an electrical analogue of the mechanical acoustic system ofthe loudspeaker; and

FIGURE 3 illustrates a modification of FIGURE 1.

Referring to FIGURE 1, the loudspeaker has an electro-magnetictransducer system including a permanent magnet 10 and a central polemember 11 located between outer magnetic members 12 and 12a. Acone-shaped piston 13 with a domed center is attached to a member 14carrying a moving coil 15. In order that the piston 13 can vibrate as arigid member, it is considerably thicker than the paper cones in commonuse and is preferably made of an expanded thermo-setting resin, e.g.foamed polyurethane, with a hard skin on its surface and having adensity between 1 and 3 lb. per cu.-ft. The effective area of the piston13 is the area normal to the direction of movement of the piston, thatis the area of a flat plane, the boundary of which is defined by theperipheral edge 13a of the piston 13. The piston 13 is carried within arigid outer support 16 by a resilient mount 17 having a compliance C Asheet of flexible material 18 extends across the region between theperiphery of the piston 13 and the outer support 16 and attached to thismaterial 18 is a rigid annular element 19 having an are-a equal to theeffective area of the piston 13 and having a mass M equal to the mass Mof the piston 13. The element 19 is conveniently a metal ring of channelsection to give the required rigidity; it is not essential however thatit should extend continuously around the annular region between thepiston 13 and the support 16 provided it has the required area and massand provided there is an air-tight seal across this region; this seal isconstituted by the flexible material 18 which provides a compliance Cbetween the piston 13 and annular element 19 and a compliance C betweenthe element 19 and support 16. The flexible material 18 has to form anair-tight seal and may for example be linen which is doped to make itnonporous, or a sheet of nylon fabric or polyvinyl chloride foil oraluminium foil. There is thus an airtight enclosure 20 within the outersupport 16, the piston 13, the flexible material 18 and the annularelement 19.

In order to obtain optimum values of the c-ompliances C and c twoseparate annular members, possibly of different thickness may beemployed. These may be corrugated to give more precise control of thesecompliances. Such an arrangement is illustrated in FIGURE 3 which showsa modification of the arrangement of FIGURE 1, two rigid annular members25, 26 being secured to the flexible material 18. FIGURE 3 also shows aflat piston 27 in place of the cone-shaped piston with a dished center 3illustrated in FIGURE 1. Apart from these distinctions, the arrangementof FIGURE 3 is similar to that of FIG- URE 1.

FIGURE 2 illustrates in conventional form the electrical analogue of themechanical-acousticsystem, Z and Z represent the load impedance of theair to the piston 13 and element 19 respectively. It will be noted thatthere are two series resonances; one is formed by M and C and C, (whereC,, is the reflected compliance provided by the air in the enclosure 20)and the other is formed by M and M with C There is also an antiresonanceformed by M with C C and C The various c-ompliances are so proportionedthat the two resonant frequencies are 1.414 and 0.707 times the nominalresonant frequency in free air of the piston 13 (discounting the mass Mof the element 19 and the compliance C It will be noted that thecompliance C loads the piston 13 at low frequencies so preventingexcessive displacement of the piston at low frequencies. The resonantfrequency of the piston is one of the main factors determining the lowfrequency response of the loudspeaker system and, by having thesefurther resonances near the nominal low frequency resonance of thepiston in free air, the low frequency response is improved.

In the construction illustrated in FIGURE 1 and in FIGURE 3, the pistonconveniently is circular in plan (form and its diameter is related tothe volume of the air in the enclosure 20, for the reasons set forth inthe specification of the aforementioned application Ser. No. 481,- 581,the preferred diameter D in inches being equal to 0.86 /V where V is theenclosure volume 20 in cubic inches.

The low frequency radiation (that is where the radiating members aresmall compared with wavelength of sound at the frequency in question) isa function of radiator area and its velocity. Because the radiationresistance is very small, Q factors of the order of 5 to 20 can easilybe realised at frequencies below 200 c./s. If therefore a number ofsubstantially rigid radiators are mechanically connected togetherthrough compliant members, a series of resonant structures will beobtained in which the resonant frequency of each radiator is a functionof its mass, the coupling compliance and the reflected reactance of theother radiators at the frequency in question. By this means it ispossible to make a small central driving diaphragm to which the voicecoil is connected and have a multiplicity of concentric rigid rings eachbeing driven by a compliance from the preceding ring. This will resultin a series of coupled resonances and by suitably proportioning theratio of mass and compliance these resonances can extend over a range ofat least two octaves, with considerably enhanced low frequencyperformance.

Referring to FIGURE 2, the analogue equivalent of a second annularmember may be considered as the provision of a further series circuitcomprising a compliance (capacitance), at mass (inductance) and a load(resistance) in shunt across M and Z A third annular member would berepresented by a similar series circuit, containing compliance, mass andload, in shunt across the mass and load of the second annular member. Inthis way further resonances can be introduced.

ill

I claim:

1. A loudspeaker comprising a fixed support, an electro-rnechanicaltransducer mounted in said support, a rigid circular piston driven bysaid transducer whereby the piston is moved with respect to the supportin accordance with applied electrical signals, a rigid annulussurrounding the piston and concentric therewith, and a sheet of flexiblematerial secured to the piston and to said fixed support, which sheetextends outwardly in a radial plane from the piston, said annulus beingsecured on said sheet of flexible material whereby the flexible materialforms a compliant coupling between the piston and the annulus andbetween the annulus and the support, the flexible material and annulustogether forming an air seal between the piston and the support, theannulus having a mass and effective area equal respectively to the massand effective area of the piston, and the spacing between the piston andannulus and between the annulus and support being so proportioned thatthe compliance between the piston and annulus is equal to the compliancebetween the annulus and the support.

2. A loudspeaker comprising a fixed support, an electro-mechanicaltransducer mounted in said support, a rigid circular piston driven bysaid transducer whereby the piston is moved with respect to the supportin accordance with applied electrical signals, at least two rigidconcentric annular members surrounding said piston, and a sheet offlexible material secured to the piston and to said fixed support, whichsheet extends outwardly in a radial plane from the piston, said annularmembers being secured on said sheet of flexible material whereby theflexible material forms a compliant coupling between the piston andinnermost annular member, between adjacent annular members and betweenthe outermost annular member and the support, the flexible material andannular members together forming an air seal between the piston and thesupport, the annular members having a mass equal to the mass of thepiston and an effective area equal to the area of the piston, and thecompliances be tween the annular members and between the outermostannular member and the support being equal to the compliance between thepiston and the innermost annular member.

3. A loudspeaker as claimed in claim 1, wherein said piston is flat.

4. A loudspeaker as claimed in claim 1 wherein said flexible materialextends over the ,whole region between the piston and the support, theannulus being fixed to the flexible material to provide the requiredmass and rigidity.

References Cited by the Examiner UNITED STATES PATENTS 1,876,831 9/1932Ballantine 18132 2,815,823 10/1957 Olson et a1. 181-32 2,863,520 12/1958Manley et al. 18131 2,905,260 9/1959 Williams l8l.--32 3,019,849 2/1962King 181-31 RICHARD B. WILKINSON, Primary Examiner.

STEPHEN ,J. TOMSKY, Examiner.

1. A LOUDSPEAKER COMPRISING A FIXED SUPPORT, AN ELECTRO-MECHANICALTRANSDUCER MOUNTED IN SAID SUPPORT, A RIGID CIRCULAR PISTON DRIVEN BYSAID TRANSDUCER WHEREBY THE PISTON IS MOVED WITH RESPECT TO THE SUPPORTIN ACCORDANCE WITH APPLIED ELECTRICAL SIGNALS, A RIGID ANNULUSSURROUNDING THE PISTON AND CONCENTRIC THEREWITH, AND A SHEET OF FLEXIBLEMATERIAL SECURED TO THE PISTON AND TO SAID FIXED SUPPORT, WHICH SHEETEXTENDS OUTWARDLY IN A RADIAL PLANE FROM THE PISTON, SAID ANNULUS BEINGSECURED ON SAID SHEET OF FLEXIBLE MATERIAL WHEREBY THE FLEXIBLE MATERIALFORMS A COMPLIANT COUPLING BETWEEN THE PISTON AND THE ANNULUS ANDBETWEEN THE ANNULUS AND THE SUPPORT, THE FLEXIBLE MATERIAL AND ANNULUSTOGETHER FORMING AN AIR SEAL BETWEEN THE PISTON AND THE SUPPORT, THEANNULUS HAVING A MASS AND EFFECTIVE AREA EQUAL RESPECTIVELY TO THE MASSAND EFFECTIVE AREA OF THE PISTON, AND THE SPACING BETWEEN THE PISTON ANDANNULUS AND BETWEEN THE ANNULUS AND SUPPORT BEING SO PROPORTIONED THATTHE COMPLIANCE BETWEEN THE PISTON AND ANNULUS IS EQUAL TO THE COMPLIANCEBETWEEN THE ANNULUS AND THE SUPPORT.